This invention relates generally to the field of aerial reconnaissance and more particularly to a method and construction of an aerial reconnaissance camera system which corrects for atmospheric dispersion. The methods are particularly advantageous for use in reconnaissance camera systems which are used to perform long range oblique photography from manned or unmanned reconnaissance aircraft. Certain aspects of the invention and methods may also have application to spaceborne cameras carried by satellites.
While it is only a thin layer in comparison to the diameter of the Earth, the atmosphere has a significant effect on the nature and quality of imagery collected while viewing through it. The atmosphere is far from benign as a transmission medium, and in fact introduces noise, attenuation, temporal variations, and aberrations into the light rays passing through it. This disclosure pertains to compensation for one of the consequences of viewing objects through this active medium, namely, atmospheric dispersion, which results in the spectral separation of imaging light rays transmitted through the Earth's atmosphere.
Refraction is defined as the bending of light rays passing from one medium to another, such as between air and water or air and glass, or between parts of the same medium with different densities such as the Earth's atmosphere. The amount of refraction is given by Snell's law and can be expressed in terms of the medium's refractive index, which is the ratio of the speed of light in a vacuum to that in the medium. Dispersion is the variation of the refractive index with the wavelength of the light.
The phenomenon of atmospheric dispersion is illustrated for purposes of explanation in FIGS. 1A and 1B. FIG. 1A shows refraction at the surface of a dispersive medium such as glass, water or air. The light coming from the left is white light, containing all colors. When it reaches the surface with an angle of incidence φ with respect to the normal to the surface (vertical line) and enters the medium M, it is bent toward the normal and makes an angle φ′ with respect to the normal. If the index of refraction inside the medium M is higher than outside, the difference (φ−φ′) is positive. The medium M in the illustration also has an index of refraction that is greater at shorter wavelengths (blue end of the visible spectrum) than at longer wavelengths (red end of the visible spectrum), and the blue light (B) is refracted through a greater angle than the yellow light (Y) and the red light (R).
The dispersive medium M could be the atmosphere of the Earth, where dispersion would refract blue light through a greater angle than the red light, as shown in FIG. 1A. The index of refraction of the atmosphere of the Earth is not constant, but rather is in proportion to the pressure, and so it decreases with increasing altitude. The path of light from a distant star through the atmosphere to a point on the ground (telescope T) is illustrated in FIG. 1B. Light approaching a surface observer from shallow elevation angles is refracted increasingly toward the vertical as it nears the observer. Its path through the atmosphere is a curved line as illustrated and blue light is deviated through greater angles than red. The difference in the angle of deviation from the actual direction (shown in FIG. 1B) is due to the refraction of the atmosphere. The difference in the apparent direction of the red and blue components of the light is due to the dispersion of the atmosphere. The dispersion created by the atmosphere blurs an image of the star taken by the telescope T, as indicated by the stellar image shown in the lower right of FIG. 1B.
In practical terms, atmospheric dispersion, if uncompensated, produces undesirable effects in reconnaissance imagery, including loss of spatial resolution and distortion of the shape of objects in imagery produced by a reconnaissance camera. In essence, the blurring shown in the lower right of FIG. 1B in the case of imaging a distant star also occurs in a reconnaissance image of a point on the Earth's surface taken from a reconnaissance camera located in an aircraft operating at high altitude.
Atmospheric dispersion as shown in FIGS. 1A and 1B has been of major interest to astronomers. Atmospheric dispersion, as a physical phenomenon, has been studied for over three centuries. Only within the last 150 years could atmospheric spectral dispersion be quantified in any significant detail. This level of detail is a result of a better understanding and measurements of the interaction of light and the atmosphere. One classical work on the subject is W. M. Smart, Text-Book on Spherical Astronomy, Cambridge University Press (1962), first edition published in 1931, which is still regarded as a definitive work. Smart illustrates two approaches to quantifying atmospheric dispersion, specifically (a) the use of the refractive invariant with a single layer model to calculate the zenith angle change through the atmosphere and (b) integrated ray tracing to find the zenith angle change. Another approach is to use the atmospheric model MODTRAN® to compute the bending angle of a line of sight from an altitude H to the ground. MODTRAN® is a computer program designed to model atmospheric propagation of electromagnetic radiation. It was developed by the US Air Force and Spectral Sciences Inc.
While methods for calculating or estimating atmospheric dispersion have been around for many years, practical methods for measurement and compensation of atmospheric dispersion have only recently evolved. These methods are mostly associated with the use of opto-mechanical and or electro-optical systems in conjunction with astronomical telescopes to provide compensation for atmospheric dispersion. One such electro-optical system employs light sensors such as photo detectors to capture incoming light rays, and an associated electronic signal processor calculates the amount spectral dispersion present. The signal processor then generates a dispersion correction signal which controls a “variable” dispersion compensating optical component placed in the optical path of the telescope. An example of a “variable” dispersion-compensating component is the Risley prism which is capable of changing its dispersion through precision movement of its optical elements which comprise two or more rotatable prisms or wedges. Therefore, when placed in the optical path of a telescope system, the optical compensating component introduces dispersion in opposition to the dispersion measured in the incoming light rays. If the amount of dispersion in the incoming light rays changes, the optic is commanded to adjust its “variable” elements to compensate accordingly. In these methods, the introduction of rotatable dispersive wedges or prisms is the principal means to compensate for dispersion.
Examples of prior art using variable optical atmospheric dispersion correction include Wallner et al., U.S. Pat. No. 4,405,203, Wein U.S. Pat. No. 5,278,402 and Takeshi et al U.S. Pat. No. 6,038,068. Wallner discloses an opto-mechanical system comprising a pair of single or a pair of compound (multiple) rotating prisms or wedges which correct for atmospheric dispersion in a telescope application. Wein, U.S. Pat. No. 5,278,402, at FIGS. 9 and 10, discloses an electro-optical dispersion correction system which includes a dispersion sensor comprising a complex multi-wavelength light detecting system. The dispersion sensor system generates a signal that is transmitted to a control circuit. The control circuit generates a control signal which drives a set of rotating prisms (a Risley prism) located within the optical system to reduce the sensed net dispersion to zero. Takeshi discloses an atmospheric dispersion correction lens arrangement for astronomical telescopes using a rotatable compound lens located within the telescope. Other related methods which compensate for aberrations caused by the atmosphere employ one or more image or light detectors to generate correction signals but do not utilize an optical aberration correcting component. Such methods rely on complex image processing algorithms that operate on frames of imagery previously captured and digitized by a camera to digitally remove the effects of the aberrations such as dispersion. One prior art example is Rhoads, U.S. Pat. No. 5,448,053.
While atmospheric dispersion correction in telescopes is known, the reconnaissance art has largely, if not entirely, ignored atmospheric dispersion as source of error and simply lived with the effects. There are several reasons for this, but the main one is that the variable optical compensator solutions disclosed in the above prior art are complex and costly, and would be extremely difficult to reliably integrate with the optical system of a current state of the art aerial reconnaissance camera.
Today's state of the art reconnaissance cameras are designed to produce extremely high resolution images, and the effects of atmospheric dispersion can no longer be ignored due to the spectral image smear present at the camera's image detector. However, apparently no practical solution for atmospheric dispersion correction in a reconnaissance camera has ever been proposed. The present invention provides a cost effective, uncomplicated, and easy-to-implement solution for compensating for atmospheric dispersion in an airborne reconnaissance camera system. As such, it is believed to represent a substantial advance in the art.